I recently installed Windows 8 Pro to an extra hard drive on my PC. My system which runs Ubuntu primarily and boots up through GRUB couldn't seem to find the new OS after I installed it. So I searched and read all of these solutions involving manually re-configuring GRUB system files, uninstalling and re-installing Windows, etc. All of these are terrible solutions. The best solution is so, so easy. All you have to do is run this update command for GRUB from a terminal window within your Linux distro:

$ sudo update-grub

It searches your hard drives for new kernels and OSs. Problem Solved. Enjoy!